1. Field of the Invention
The present invention relates generally to media sheet feeders for hard copy machines, such as printing and plotting devices, more particularly to cut-sheet picking mechanisms for media sheet feeders and more specifically to a pre-pick device.
2. Description of the Related Art
Many office products such as computer printers and plotters, plain paper facsimile machines, and photocopiers use mechanisms that feed a single sheet of pre-cut printing medium (for example, a sheet of paper of a particular size such as standard letter size, legal size, or A-4 (metric), or transparencies, or envelopes) into the hard copy producing apparatus. These mechanisms are commonly referred to as "sheet feeders."
Sheet feeders usually are provided with an adjustable or replaceable media cartridge, tray, or other type of stacker in which a user can stack multiple cut sheets of the media. The use of cartridges adapted to the various styles of media provide a mechanism for quick changes between any particular printing medium by the user.
Sheet feeders generally provide a corner separation mechanism which holds down the lead corners of the stack of media which is biased, such as by spring loading, in the opposing direction. Upon receiving a FEED command from the hard copy machine controller electronics, a sheet picking device is actuated to deliver the top sheet from the stack over the resistance of the corner separation mechanism. As shown in FIG. 1, one popular mechanism for feeding the top sheet from the stack is a D-shaped roller 2 with a D-rim mounted friction member 10. This device is commonly referred to in the art as a "pick roller."
A pick roller 2 generally includes relatively flat, side plates 6, 8 and a radial, friction member 10 mounted (or integrally fabricated) on the D-rim shaped portion of the pick roller 2 between the side plates 6, 8. The pick roller 2 is mounted on a drive shaft 3 through a mounting aperture 4 connected to a drive motor 5. Upon a FEED command, the pick roller 2 rotates around its drive shaft 3 and the friction member 10 (such as a rubber pad (or "tire" section), cork, grit wheel, or other friction providing material) comes into contact with the surface of the top sheet of the counter-biased stack of media. The pick roller 2 is designed to generate enough downward force against the stack so that the force between the friction member 10 and the stack bias drives at least a top sheet of the stack forward. As the sheet moves forward, it must overcome the corner separator. Once the friction is sufficient to overcome the corner separator, the corner of the sheet snaps over the separator and the pick roller 2 moves the sheet into the hard copy machine where other transport mechanisms can take over.
One problem with the prior art is that pick rollers have a narrow range of media weight and type that will feed reliably. For example, with very light weight paper, the force of the pick roller against the biased media stack is transmitted to sheets of paper below the top sheet, causing more than one sheet of paper to snap over the corner separator. Picking multiple sheets is a common failure and usually requires the operator to halt operations to remove the extra sheets and to clear a resultant paper misfeed, or paper jam, error message from the machine.
Therefore, there is a need to expand the range of accurate, single sheet medium feed capability for hard copy machines.